By looking for discrepancies in Uranus's orbit, Englishman John Couch Adams and Frenchman Urbain Le Verrier each independently calculated the unseen world's mass and position.

But Neptune's existence wasn't confirmed until September 23, 1846, when German astronomer Johann Galle used a telescope to search the predicted location in the sky and spotted a tiny blue-green disk. The planet was eventually named Neptune, after the Roman god of the sea.

Ironically, Galileo Galilei spotted Neptune more than 200 years earlier but wrongly assumed the planet was just a star.

"He observed it and he thought it moved relative to background stars, but it became cloudy and he was unable to observe it further," said Rocky Alvey, director of the Vanderbilt Dyer Observatory in Brentwood, Tennessee.

"If it hadn't been for clouds, Galileo may have been the discoverer of Neptune."

The closest approach to Neptune by a spacecraft occurred in 1989, when NASA's Voyager 2 passed within 3,000 miles (4,950 kilometers) of the planet's north pole.

But even with more than a century and a half of observations, many unresolved questions remain about the farthest planet from the sun.

For example, it's still unclear how Neptune is generating the heat needed to power the changes astronomers have observed in its atmosphere.

"It only gets 0.1 percent as much sunlight as Earth, but Neptune has storms and its appearance changes over time," said Erich Karkoschka, an astronomer at the University of Arizona.

Neptune was much darker in the 1970s and '80s than it is now, and large dark spots corresponding to giant storms have previously been observed in the planet's atmosphere by Voyager 2 and the Hubble Space Telescope.

"If we can understand how a planet with so little sunlight can have a very active atmosphere," Karkoschka said, "then I think we'll understand Earth's atmosphere and the planetary atmospheres of the more than 500 exoplanets around other stars better."